When alkaline spent pulping liquors are burnt to yield chemicals and heat, one of the main products is sodium carbonate. In the case of black liquor from kraft cooks, sodium sulfide will also be formed. The product is dissolved in water into so-called green liquor after the passage through the recovery furnace. As a rule, the carbonate is usually not sufficiently alkaline to pulp wood or similar fibrous material to an adequate degree. The carbonate is consequently transformed into hydroxide. This process is called caustisation, and is conducted with the aid of a metal hydroxide solution, of which the corresponding carbonate has a low solubility in water. In practice, calcium hydroxide is used for the caustisation. In addition to soluble sodium hydroxide, insoluble calcium carbonate is also formed (lime sludge), which is usually separated, heated (lime sludge reburning), until it has been transformed into calcium oxide and is then dissolved into new calcium hydroxide. Caustisation requires both equipment and time, and if it could be avoided this would mean a considerable saving for a pulp mill.